1. Field of the Invention
The present invention relates to an omni-directional reception sensitivity three-axis antenna which is used in a receiving device of a keyless entry system for locking or unlocking a vehicle, etc.
2. Description of the Related Art
As an antenna for LF band, a bar antenna which consists of wire wound around a bar-type core winding axis is used. Such a bar antenna has a reception sensitivity in the direction of the winding axis and does not have that in directions orthogonal to the winding axis. Therefore, plural antenna coils mutually compensate for their respective area lacking reception sensitivity by arranging three antenna coils such that the respective winding axes orthogonally cross each other, an omni-directional antenna having omni-directional reception sensitivity is obtained.
In recent years, a small-sized three-axis antenna, having three coils wound orthogonally to each other around a single core, as shown in Japanese patent laid-open No. 2004-15168, is used widely.
FIG. 15 shows an example of a prior art three-axis antenna. As shown in FIG. 15, a conventional three-axis antenna 70 is configured by a core 80 consisting of an externally flat disk-type ferrite core 80, on which circumference surface, mutually orthogonally crossing on the top and bottom surface of the core 80, an x groove 81, a y groove 82 and a z groove 83 are provided, with an x axis coil 91, a y axis coil 92 and a z axis coil 93 are respectively wound around the x groove 81, the y groove 82 and the z groove 83.
The three-axis antenna 70 has omni-directional reception sensitivity due to the winding axes of the x axis coil 91, the y axis coil 92 and the z axis coil 93 being orthogonal to each other.